can anyone explain me... why use wires instead of a perforated metal plate?

I can think of a few reasons...there may be others.

1) Wires allow you to minimize the capacity of the panel by only putting wires over the driven area of the diaphragm. Most perforated metal ESLs have the metal extending on to the spacers where they are supported and attached. The dielectric constant of the spacers are usually ~3 times that of air, so this can add considerable "wasted" capacitive load to the panel. Wasted, because the amplifier has to drive it, but it is not used to produce sound. The lower the capacitive load of the panel, the higher the step-up ratio of your transformer can be for the same high frequency cut off, and similar load on the amplifier. ie. higher efficiency.

2) Wires allow you the flexibility to modify the directivity and radiation pattern vs frequency by segmentation. This can reduce beaming of the high frequencies from a large panel.

3) Most builders will find it much easier to build "arc-proof" panels using PVC insulated wires than with perforated metal. Personally, I have found it very difficult, if not impossible, to properly coat around the sharp edges of the perforation holes. This may not be a problem if high resistance coating is used on the diaphragm, and the transformer step up ratio is not too high. But, when trying to maximize efficiency with high step up ratios, or high power solid state amplifiers, the sharp edges around the perforation holes will promote corona and arcing unless properly coated.

right so far, but to be precise, it´s not at all desirable to use high transformation factors U. Better is to use only as much U as needed and as low U as possible.
Lower capacitance values mean higher impedance levels, means higher transformation factors needed to match amplifier and panel, means higher voltage levels needed to drive the capacitance, means more losses in the tranny because of thicker insulation, means lower dynamics, means a loss in sonic quality of the tranny, means a loss in safety margin, means a loss in lifetime and so on and so on.
High U-Values are rather a sign of inefficient panels. Even though most metal sheet panels ´suffer´ from a higher percentage of ´dead´ or ´wasted´ capacitance, they are in fact more efficient than wire panels. Typically transformation factors for wideband sheet metal panels are around 60-80, sometimes even lower. Wire panels of similar dimensions usually need Us of >100. Segmented wire panels reduce the active membrane area with rising frequency. This allows to control distribution character and frequency response to a certain degree and leads to a slightly different sonic fingerprint to a flat metal sheet panel or curved metal sheet panels.

The real decicive points are #3 and #4. Ease of building and material supply -with a good chance of success with the first try- and low cost is what speaks for wire.

...Even though most metal sheet panels ´suffer´ from a higher percentage of ´dead´ or ´wasted´ capacitance, they are in fact more efficient than wire panels...

I would agree that "typical" wire ESLs tend to be less efficient than "typical" PSM(perforated sheet metal) ESLs. I believe that this is mainly due to the fact that most wire ESLs are built with larger spacing and segmentation. As you mentioned, both reduce the efficiency.

Before pursuing wire ESLs, I built and compared two panels(one wire, one PSM) of identical size and spacing (48" x 6" with 1/16" spacing and 45% open area). Driven without segmentation, the same bias voltage, and the same transformers(80:1 ratio), the wire ESL was identical in SPL to the PSM ESL at 10Vrms input. At 28Vrms, the wire ESL was, perhaps, +1.0dB more efficient. I can't recall the exact panel capacities, but it was roughly 500pF for the wire ESL, and 700pF for the PSM. This resulted in the impedance at 20kHz being about 0.5 ohm lower for the PSM.

I'm not sure of the dimensions of the chicken fencing you're considering, but the fencing I'm familiar with would be terrible for an ESL panel. Maybe I'm just envisioning the wrong material. How big is the wire and what is the size of the holes? You probably don't want to deviate too far from a 50% open area (maybe plus or minus 10%).

I'm building wire ESLs primarily because I want to control the directivity of the speakers. I love the dynamics of the flat perforated metal ESLs I've been living with for a decade, but the directivity pattern isn't one I want to continue living with. Others might have a different set of priorities, and perforated metal might make more sense for them. The perf. metal panels are certainly easier to build if you're not too picky about the insulation. If you want reliable insulation, well, that's another situation entirely. I've enjoyed listening to panels with less than state of the art insulation, but I'm sure I could improve things with more careful attention to the insulation. (I used epoxy paint as insulation on perforated steel panels.)

the results in Matthew Lattis´s Report in AudioXpress 2005 says the opposite.
He found too that PSMs (perforated sheet metal stators) typically gave higher efficiency values. Admittedly it is difficult to compare in a ´truely comparable´ way, since there are several factors affecting efficiency which are hard to control or difficult to change without affecting other parameters as well.
For example what value is chosen for d/s? Whilst with SMS there is a plain with holes in --> easy measurement, because of constant d/s. But with round shaped wires of a WS (wire stators) You probabely have to calculate with a ´effective´ d/s value which is larger than the measured d/s. Next the insulation wires is typically a bit thicker than the insulation thickness of SMS. Next there are usually differences in the material choice of insulation and as such the epsilon-value of he insulation material itself. The open area %age is affected by this too. Really high precision built of the panels is needed with very small dimensional deviations, because even small variations affect very much the outcome.

In practise You find the more efficient panels (lower U for same SPL) amongst the PSM-hybrids.